Most combustion engines have fuel systems that deliver fuel to the engine in metered amounts, where the fuel delivery may be controlled by an engine management system. Gas turbine engines, such as auxiliary power units (APUs), are used as an example herein, but those skilled in the art will recognize that the systems and methods disclosed herein may be used with any fuel delivery system. APUs are typically used by aircraft to provide electrical and/or pneumatic power to aircraft for various uses. A typical APU consists of a gas turbine engine that rotates at high speed during operation. To prevent overspeed of the engine, the APU is designed to not only control speed but to also have redundant shut down capability. Such redundant shut down capability consists of a minimum of two separate systems, each capable of shutting down the APU. Each independent system must not have a latent failure potential that in and of itself could lead to overspeed of the APU from a single event failure.
APUs typically contain a fuel metering valve to meter fuel flow to the APU's combustor and one or more fuel shutoff solenoid valves to allow or prevent fuel flow. If the fuel metering valve sticks in the open position, an APU overspeed condition could result. To prevent overspeed in this case, the fuel shutoff solenoid valve must be commanded closed. To prevent latent failure of the fuel shutoff solenoid valve, proper operation of the fuel shutoff solenoid valve is typically checked during each APU commanded shutdown.